The metallicities and kinematics of RR Lyrae variables, 1: New observations of local stars

Abstract

In order to study the structure and formation history of the galaxy, we have obtained low-to-moderate dispersion spectra of 302 nearby RR Lyrae variables of Bailey type 'ab'. We derived abundances, typically accurate to 0.15-0.20 dex and calibrated to the Zinn & West (1984) globular cluster metallicity scale, from the pseudoequivalent widths of the Ca II K, H delta, H gamma, and H beta lines. Radial velocities accurate to between 2 and 30 km/s were obtained from the spectra and from the literature. Distances accurate to between 5% and 20% were derived from published apparent magnitudes and Burstein & Heiles (1982) reddenings. The metallicity distribution of the RR Lyrae stars peaks at (Fe/H)K approximately equals -1.5, and is narrower than that of the Ryan & Norris (1991) subdwarfs, as expected since the most metal-rich and metal-poor progenitors preferentially appear as stable red and blue horizontal branch stars, rather than as RR Lyrae. The metal-rich tail of the RR Lyrae distribution extends to (Fe/H)K approximately equals 0, and a qualitative analysis of the distribution of distances from the galactic plane shows that the stars in this tail (i.e., (Fe/H)K greater than -1.0) are more concentrated to the plane than the more metal-poor stars. The abundance distribution of the local RR Lyrae stars is in excellent agreement with the changing abundance distributions of distant RR Lyrae stars as a function of galactocentric distance, as derived by Suntzeff et al. (1991), who ascribed this change to systematic variation in horizontal branch morphology (probably age variations) with galactocentric distance. The abundance distribution of the local RR Lyrae stars also agrees well with those of the distant RR Lyrae stars as a function of distance from the galactic plane. There is no evidence for an abundance gradient in this direction, suggesting that gaseous dissipation did not play a major role in the formation of the outer halo.